Control systems are becoming increasingly important in all sectors of industry, such as process control, flexible manufacturing operations, total quality initiatives, assembly lines, machine tool controls, computer systems, and robotics. One of the strongest movements in the development of control systems is the movement toward xe2x80x9copenxe2x80x9d systems.
An xe2x80x9copenxe2x80x9d system is a system designed to provide interoperability between devices made by different vendors. Interoperability is the ability of field devices made by different vendors to communicate and work together. The movement toward open systems was initiated in the 1970s and early 1980s with the development of the Open System Interconnect (OSI) network model. The OSI network model is a seven layer network architecture, including a physical layer, data link layer, network layer, transport layer, session layer, presentation layer, and application layer.
However, the OSI network model, and other existing network models, are insufficient to provide interoperability. Today, devices made by different manufacturers usually cannot communicate without bridging technologies, such as gateways and translators. For example, in U.S. Pat. No. 5,526,358, issued Jun. 11, 1996, communication between field devices using a different transport protocol stack requires a dynamic context bridge. Thus, what is needed is a system which offers interoperability with devices made by different vendors.
The movement toward open systems is driven by vendors and users. Vendors want open systems because open systems would allow them to sell their product to more users, instead of only to the users operating a specific proprietary system. Users want open systems so that they can use the best field devices in their systems, not only the field devices specifically designed for a proprietary system.
Two common network control method are: xe2x80x9ctoken passingxe2x80x9d and xe2x80x9cmaster for a moment.xe2x80x9d With the xe2x80x9ctoken passingxe2x80x9d system, any device with a ready message may, upon receiving a control token, send a message to another devices. A token passing control system is disclosed in U.S. Pat. No. 4,888,726, issued Dec. 19, 1989. With the xe2x80x9cmaster for a momentxe2x80x9d system, each device is adapted to accept and then relinquish supervisory control of the bus. Specifically, once a field device assumes supervisory control, the field device sends data or status requests to other field devices. When the field device is finished sending data and status requests, the field device forwards supervisory control to another device. A xe2x80x9cmaster for a momentxe2x80x9d system is disclosed in U.S. Pat. No. 4,347,563, issued Aug. 31, 1982.
There also has been a trend toward distributed control systems. Distributed control systems are an alternative to central control systems. In central control systems, a central controller performs all the control functions. In distributed control systems, more than one field device operating in the system takes an active role in the control functions.
However, the current distributed control systems do not synchronize application processes within the field devices with the network processing. As a result, these systems commonly experience xe2x80x9cjitter.xe2x80x9d xe2x80x9cJitterxe2x80x9d is when an application process does not occur at its scheduled time according to the network processing. Thus, what is needed is a system capable of synchronizing the application processes with the network processing.
What is needed is a control system that provides greater manufacturing flexibility, increased productivity, higher product quality, and improved operator and plant safety.
What is needed is a distributed control system that supports high speed applications and remote I/O.
What is needed is a distributed process control system that detects and reports its own errors, alerting operators to problems on a xe2x80x9creal-timexe2x80x9d basis.
What is needed is a distributed control system with synchronized applications processing and network processing.
The present invention overcomes the shortcomings described above and provides a new and improved block oriented control system. The present invention is a system for distributing the control within process control systems having intelligent field devices capable of data processing and management. The intelligent field devices are usually connected by a bus, which is a digital, serial, two-way communication system. A field device is any type of equipment used to remotely collect data or provide a response in a process control system, such as sensors, actuators, and motors. A field device is intelligent if it incorporates a program interface comprising three main functional components: 1) a physical layer; 2) a communication stack; and 3) a user layer. These layers are derived from the Open Systems Interconnect (OSI) model. To incorporate these layers, a field device generally includes a processor, memory, and medium attachment unit, such as a network adapter.
The physical layer is the same as OSI layer 1. The physical layer of the device receives messages from the communications stack and converts the message into physical signals on a transmission medium and visa-versa. The transmission medium could be a bus or two-way wiring. The physical layer is defined by approved standards from the International Electrotechnical Commission (IEC) and the International Society of Measurement and Control (ISA).
The communication stack generally comprises a data link layer, a fieldbus message specification, a fieldbus access sublayer and a network management layer. These layers correspond to OSI layers 2 and 7. The data link layer (DLL) is the same as OSI layer 2. The data link layer controls transmission of messages onto the bus based on the commands from a Link Active Scheduler (LAS). The link active scheduler is the intelligent field device that is assigned the responsibility of acting as a network controller. Other intelligent field devices capable of assuming the responsibility of the central controller, but not actually assigned the responsibility, are called Link Master Devices.
The fieldbus message specification and the fieldbus access sublayer are sublayers within OSI layer 7. The fieldbus message specification provides a standardized way for the user layers to communicate over a bus. Specifically, the fieldbus message specification describes the communication services (e.g., read, write, abort, etc.), message formats, and protocol behavior to build messages for the function block applications, and contains an object dictionary. The object dictionary contains descriptions of all the parameters available in that field device. The description of these parameters may be written in a variety of computer languages, such as C++ or Smalltalk, but preferably it is written in the Device Description Language.
The.fieldbus access sublayer maps the fieldbus message specification to the data link layer, and thus, provides the needed functionality of OSI layers 3-6. The network management coordinates configuring the communications stack, performance monitoring, and fault detection monitoring.
The distributed control system is implemented by the user layer. The user layer includes 1) function blocks (FBs) and 2) system management. The function blocks are standardized encapsulations or blocks of control functions, such as analog inputs and outputs, discrete inputs and outputs, proportional/derivative control and proportional/integral/derivative control. In the preferred embodiment, there are ten standard function blocks for basic control and an nineteen standard function blocks for advanced control. Together, these twenty-nine standard function blocks provide most of the functionality required to build a field device for a manufacturing or process control system. For example, a temperature transmitter may contain an analog input function block, and a control valve may contain a proportional/integral/derivative function block and an analog output block. A complete control loop can be built using only a sensor and a valve since these devices contain an analog input, a proportional/integral/derivative, and analog output function blocks.
To coordinate the execution of the function blocks, the field device should also include system management. The system management provides the time synchronization for scheduled execution of function blocks while the link active scheduler schedules communication of function block parameters on the bus. System management also handles other system features such as broadcast of universal time throughout the system and automatic device address assignment.
These and other advantages of the invention are achieved, for example, by a block-oriented distributed process control system that includes a transmission medium and a plurality of field devices, connected to the transmission medium. Each field device includes a user layer, which includes a function block application that defines the functionality of the field device, a physical layer, which translates messages from the transmission medium into a suitable format for the user layer and from the user layer into a signal for transmission on the transmission medium, and a communication stack, connected to the user layer and the physical layer. The communication stack includes a data link layer and an application layer and the data link layer controls the transmission of messages onto the transmission medium and the application layer allows the user layer to communicate over the transmission medium.
These and other advantages of the present invention are also achieved by, for example, a field device capable of operating in an open control system. The field devise includes a memory that includes system management data. The system management data includes a system schedule. The field device also includes a function block, that includes parameters and a computer program, a resource block, that makes hardware specific characteristics of the device electronically readable, and at least one transducer block that controls access to the function block. The field device also includes a processor, operably connected to the memory, that executes the function block based on the system schedule, and a medium attachment unit, which translates input messages and output messages between the processor and a transmission medium.